With the recent advancements in the information telecommunication technology, the speed of optical transmission, the capacity thereof and the transmission distance thereof have been increasing, thus requiring a very advanced transmission technology. For example, one form of the optical transmission technology is the PON (Passive Optical Network) system. In this system, one switching office is connected to a plurality of subscriber premises based on time division multiplexing. Also with the transmission between the switching office and the subscriber premises, the distance and speed of data transmission have been increasing. Long-distance transmission requires a high power level to be transmitted, and it is therefore necessary to drive large currents. This increases the driving transistor size, thereby also increasing the load capacitance of the transistor and wires connected thereto. As a result, it is no longer possible to achieve a sufficient rising/falling response, and a fast response is difficult to realize. Where a receiver receives transmitted data, if data of insufficient rising/falling characteristics is received, pattern effects occur and the data quality may degrade through, for example, dropping of data.
In the prior art, optical transmitter circuits for transmitting data primarily employed the single driving method being superior in terms of the power consumption for low-speed communications. However, as the communications speed increases to 622 Mbps, 1.25 Gbps, . . . , the differential driving method is becoming the mainstream.
Under such circumstances, as an example of a measure for shaping the waveform of transmitted data, a technique is known in the art to control the amplitude of the input voltage to the laser diode driving differential switch transistors in connection with the amplitude of the modulation current to the laser diode (see Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 5-218543